The present invention relates to an energy storage subsystem for vehicles, comprising:                a metal casing,        an electrical storage system mechanically fastened in the metal casing, and comprising at least one supercapacitor module, the or each module comprising a metal enclosure and a plurality of supercapacitors linked together in series and arranged in the metal enclosure.        
Such an energy storage subsystem is notably designed to recover and store the braking energy of vehicles, in particular of rail vehicles travelling on a network. It can be installed permanently at a fixed point of the network or onboard one of the rail vehicles, the energy being in both cases stored in supercapacitors of the or each module of the subsystem.
It is commonplace for such a storage subsystem to comprise a network of supercapacitor modules connected in series, in order to obtain a significant energy reserve for the rail vehicle. When the storage subsystem is onboard a vehicle, the lowest potential of the network of modules is linked to the mechanical ground of the vehicle. Because of this, there is a significant voltage between the electrical ground of the vehicle and the internal elements of the modules furthest away from the mechanical ground, which exhibit a high electrical potential, for example roughly equal to 400 V. In practice, when the enclosure of the modules is metal, each of the modules being mechanically fixed to the mechanical ground, the voltage between this metal enclosure, exhibiting an electrical potential to the ground, and the internal cells of the module, is high. This high voltage does not however result in any flow of high currents in the modules, likely to damage the latter, because an insulating blanket is arranged between the enclosure of each module and the elements internal to the module.
However, in the case of failure of the internal insulation of a supercapacitor module, a short circuit to ground occurs with a very high current which is likely to result in the complete destruction of the module and a release of gas which is hazardous, even toxic, in the environment close to the module. This release of gas is due to the presence of electrolyte inside the supercapacitors, the electrolyte being volatilized when a current passes through the enclosure of the module, in the event of piercing for example.